Circular tubes have been used in many fields. For example, circular glass tubes have been used for electric lamps such as a fluorescent lamp or for electric heaters. Similarly, circular plastic tubes have been used in industrial machinery and tools. The circular tubes are generally shaped from elongated tubes.
Conventionally, an apparatus for shaping an elongated tube, e.g., an elongated glass tube, into circle has used a shaping drum. The shaping drum is formed with a round groove corresponding to the thickness of the elongated tube.
In a shaping process according to the conventional apparatus, a whole length of an elongated glass tube is initially softened by heat. The elongated glass tube thus softened is wound around the shaping drum. Thus, a circular glass tube is obtained.
The conventional apparatus requires a large scale furnace for first heating the whole length of the elongated glass tube. The elongate glass tube softened by the furnace has to be wound around the shaping drum very quickly. Thus, the apparatus is high in cost and the shaping operation of the glass tube is very difficult. Further, a plurality of shaping drums with different diameters must be prepared for manufacturing circular glass tubes with different diameters.
To eliminate the drawbacks of the above conventional apparatus, another conventional apparatus comprising a turning arm, as shown in the Japanese Patent; Tokko-sho P57-8770 issued on Feb. 18, 1982, has been developed. Now, the second conventional circular glass tube shaping apparatus and a circular glass tube shaping process according to the apparatus will be explained in reference to FIGS. 1 and 2. FIGS. 1 and 2 are elevations schematically showing the second conventional circular glass tube shaping apparatus in which an elongated glass tube is initially set to the apparatus and the glass tube is finally shaped into circle by the apparatus, respectively.
As shown in FIGS. 1 and 2, the conventional circular glass tube shaping apparatus comprises a front support 10, a rear support 11, a heater 12, a turning arm 13 and a turn drive motor 14 for driving an elongated glass tube 15a to be shaped into a circle. The front support 10 and the rear support 11 support the front and the rear of the elongated glass tube 15ai.e., the rightward portion and the leftward portion of the elongated glass tube 15a in the drawing, respectively.
The front support 10 is mounted at a fixed position in the apparatus and allows the elongated glass tube 15a to slide along its axis in the rightward direction. The rear support 11 is movably mounted in the apparatus so that the rear end of the elongated glass tube 15a is always supported thereby during movement. The rear support 11 has a rear chuck 16 for holding the rear end of the glass tube 15. The heater 12 locally heats a portion of the glass tube 15a ahead of the front support 10. The heater 12 is constituted by, e.g., a gas burner, an electric heater, a laser heater and etc. The turning arm 13 has a front chuck 17 on its free end. The front chuck 17 holds the front end of the glass tube 15 ahead of the heater 12. The turn drive motor 14 turns the turning arm 13 around an axis 18 on the othe rend of the turning arm 13 in the clockwise direction in the drawing.
The elongated glass tube 15a is cut to a given length Lp corresponding to a circular length of a resulted circular glass tube 15b (see FIG. 2), prior the elongated glass tube 15a being set in the apparatus.
When the elongated glass tube 15a is set in the apparatus, as shown in FIG. 1, the heater 12 starts to locally heat a portion of the glass tube 15a near the front support 10. The turn drive motor 14 turns the turning arm 13 in the clockwise direction when the locally heated portion of the glass tube 15a is softened. Thus, the front chuck 17 of the turning arm 13 pulls the glass tube 15 rightward during its clockwise circular motion. The heat softened portion of the glass tube 15 is bent in response to the circular motion of the turning arm 13 and then hardened. The circular motion of the turning arm 13 ends when the turning arm 13 has turned by a given angle .theta.1 in which the front chuck 17 is prevented from further circular motion by the front support 10 or the rear support 11 (see FIG. 2). As a result, the rear of the circular glass tube 15b thus shaped is left straight.
As explained above, the conventional circulr glass tube shaping apparatuss can not shape the whole length of the elongated glass tube 15a into a circle. Thus, the resulting circular glass tube 15b is unattractive.
If it is desired to improve the appearance of the resulting circulr glass tube 15b, the straight rear portion is cut out after the shaping process. The elongate glass tube 15a must then be initially cut to an extended length Le which is longer than the given length Lp by a predetermined surplus length Ls corresponding to the rear portion of the glass tube 15 left straight at the end of the shaping process, prior the elongated glass tube 15a being set in the apparatus. The three lengths have a relation Le=Lp+Ls. In this case, the circular glass tube shaping process requires the extra process for trimming the straight rear portion. The straight rear portion thus trimmed is wasted. In addition, the diameter of the circular glass tube 15b becomes large as compared to the former case where the straight rear portion is left as the part of the resulting circular glass tube 15b.